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In
petroleum geology Petroleum geology is the study of origin, occurrence, movement, accumulation, and exploration of hydrocarbon fuels. It refers to the specific set of geological disciplines that are applied to the search for hydrocarbons ( oil exploration). Sedim ...
, a trap is a geological structure affecting the
reservoir rock A petroleum reservoir or oil and gas reservoir is a subsurface accumulation of hydrocarbons contained in porous or fractured rock formations. Such reservoirs form when kerogen (ancient plant matter) is created in surrounding rock by the presence ...
and
caprock Caprock or cap rock is a more resistant rock type overlying a less resistant rock type,Kearey, Philip (2001). ''Dictionary of Geology'', 2nd ed., Penguin Reference, London, New York, etc., p. 41.. . analogous to an upper crust on a cake that is ha ...
of a petroleum system allowing the accumulation of hydrocarbons in a
reservoir A reservoir (; from French ''réservoir'' ) is an enlarged lake behind a dam. Such a dam may be either artificial, built to store fresh water or it may be a natural formation. Reservoirs can be created in a number of ways, including contr ...
. Traps can be of two types: stratigraphic or structural. Structural traps are the most important type of trap as they represent the majority of the world's discovered petroleum resources.


Structural traps

A structural trap is a type of geological trap that forms as a result of changes in the structure of the subsurface, due to
tectonic Tectonics (; ) are the processes that control the structure and properties of the Earth's crust and its evolution through time. These include the processes of mountain building, the growth and behavior of the strong, old cores of continents ...
, diapiric, gravitational and compactional processes.


Anticlinal trap

An
anticline In structural geology, an anticline is a type of fold that is an arch-like shape and has its oldest beds at its core, whereas a syncline is the inverse of an anticline. A typical anticline is convex up in which the hinge or crest is t ...
is an area of the subsurface where the strata have been pushed into forming a domed shape. If there is a layer of impermeable rock present in this dome shape, then hydrocarbons can accumulate at the crest until the anticline is filled to the ''spill point'' - the highest point where hydrocarbons can escape the anticline. This type of trap is by far the most significant to the hydrocarbon industry. Anticline traps are usually long oval domes of land that can often be seen by looking at a geological map or by flying over the land.


Fault trap

This trap is formed by the movement of
permeable Permeability, permeable, and semipermeable may refer to: Chemistry *Semipermeable membrane, a membrane which will allow certain molecules or ions to pass through it by diffusion *Vascular permeability, the movement of fluids and molecules betwe ...
and impermeable layers of rock along a fault plane. The permeable reservoir rock faults such that it is now adjacent to an impermeable rock, preventing hydrocarbons from further migration. In some cases, there can be an impermeable substance smeared along the fault surface (such as clay) that also acts to prevent migration. This is known as clay smear.


Stratigraphic trap

In a stratigraphic trap, the geometry allowing the accumulation of hydrocarbons is of sedimentary origin and has not undergone any tectonic deformation. Such traps can be found in clinoforms, in a pinching-out sedimentary structure, under an
unconformity An unconformity is a buried erosional or non-depositional surface separating two rock masses or strata of different ages, indicating that sediment deposition was not continuous. In general, the older layer was exposed to erosion for an interval ...
or in a structure created by the creep of an
evaporite An evaporite () is a water- soluble sedimentary mineral deposit that results from concentration and crystallization by evaporation from an aqueous solution. There are two types of evaporite deposits: marine, which can also be described as ocean ...
.


Salt dome trap

Masses of salt are pushed up through clastic rocks due to their greater buoyancy, eventually breaking through and rising towards the surface (see
salt dome A salt dome is a type of structural dome formed when salt (or other evaporite minerals) intrudes into overlying rocks in a process known as diapirism. Salt domes can have unique surface and subsurface structures, and they can be discovered usi ...
). This salt is impermeable and when it crosses a layer of permeable rock, in which hydrocarbons are migrating, it blocks the pathway in much the same manner as a fault trap. This is one of the reasons why there is significant focus on subsalt imaging, despite the many technical challenges that accompany it.


Hybrid trap

Hybrid traps are the combination of two types of traps. In the case of
tilted blocks Tilt may refer to: Music * Tilt (American band), a punk rock group, formed in 1992 * Tilt (British band), an electronic music group, formed in 1993 * Tilt (Polish band), a rock band, formed in 1979 Albums * ''Tilt'' (Cozy Powell album), 1981 ...
, the initial reservoir geometry is the one of a fault-controlled structural trap but the
caprock Caprock or cap rock is a more resistant rock type overlying a less resistant rock type,Kearey, Philip (2001). ''Dictionary of Geology'', 2nd ed., Penguin Reference, London, New York, etc., p. 41.. . analogous to an upper crust on a cake that is ha ...
is generally made by the draping sedimentation of mudstones during the oceanisation process.


See also

*
Petroleum reservoir A petroleum reservoir or oil and gas reservoir is a subsurface accumulation of hydrocarbons contained in porous or fractured rock formations. Such reservoirs form when kerogen (ancient plant matter) is created in surrounding rock by the presenc ...
*
Structural geology Structural geology is the study of the three-dimensional distribution of rock units with respect to their deformational histories. The primary goal of structural geology is to use measurements of present-day rock geometries to uncover informati ...


References

{{reflist Petroleum geology